default or selected kernelversion does not have config value CONFIG_CLK_RENESAS_LEGACY.
Result is shown for kernelversion 6.9.9

Legacy DT clock support

configname: CONFIG_CLK_RENESAS_LEGACY

Linux Kernel Configuration
└─>Device Drivers
└─>Common Clock Framework
└─>Legacy DT clock support
In linux kernel since version 4.2 (release Date: 2015-08-30)  
Enable backward compatibility with old device trees describing a
hierarchical representation of the various CPG and MSTP clocks.

Say Y if you want your kernel to work with old DTBs.
It is safe to say N if you use the DTS that is supplied with the
current kernel source tree.